How a Stopcock Works

Shown: Teflon Stopcock

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A teflon stopcock (i.e., plug is teflon - but barrel is glass) is shown disassembled. The wooden swab inserted through the through the teflon plug in the photo illustrates that in these and similar ball-type valves (including all-glass stopcocks and various metal valves), the hole is conventionally aligned parallel to the handle (orange bar). This is so you can tell at a glance whether a valve is open.


Teflon stopcocks require no grease.


NOTE:  Glass stopcocks (glass plug and glass barrel), not illustrated) are similar in design but contact surfaces are ground glass and these must be adequately greased to prevent leakage. With glass valves, a balance must be struck between enough grease for a seal versus so much the hole gets plugged. When in doubt, ask (we sometimes check out large separatory funnels with glass stopcocks, e.g., Experiment G - Oil of Cloves).

(In the research lab, glass stopcocks are often the valve of choice for precision work, since when properly greased they provide an excellent seal.)

©2001,2002 Daniel A. Straus

An assembled teflon stopcock is shown. On a high quality monitor, the hole can be seen in the (closed) plug. The (orange) nut should be snug, but not overly tight, for a good seal which does not bind (plug turns smoothly).


NOTE: When performing a liquid-liquid extraction, pouring liquid into a separatory funnel with an open stopcock is a very embarrassing maneuver, and usually leads to starting the experiment over.....

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